An improved load indicating fastener is utilized to indicate when a predetermined preload has been applied to the fastener to thereby apply a force against a member and to indicate when the force against the member is reduced.
When a fastener, such as a nut or bolt, is subjected to a varying load, fatigue failure tends to occur. The endurance strength and fatigue life of a fastener are greatly enhanced by application of a precise preload to the fastener. Although torque measuring devices, such as torque wrenches, have been used to measure the extent of the preload applied to a fastener, this method of determining preload is inaccurate due to variations in the coefficient of friction between mating surfaces of a fastener. It has been estimated that the preload applied to a fastener will vary by plus or minus 30% when torque is used as a measure of preload.
Washers have been associated with fasteners to provide a visual indication when a predetermined clamping force is applied against the washer. The use of these load indicating washers provides an indication of when a desired preload has been applied to a fastener. Known load indicating washers are disclosed in U.S. Pat. Nos. 3,948,141; 3,306,154; and 3,304,827.
In addition to the aforementioned load indicating washers, there are known washers having retaining elements to hold the washer against rotation relative to a fastener and/or a member against which the washer is pressed by the fastener. At least some of these washers have waves or undulations. When the undulating washers are pressed against the member, the undulations tend to be flattened and the retaining elements are forced into engagement with the member. Some of these known washers are disclosed in U.S. Pat. Nos. 3,390,713; 3,077,218; 2,179,575; and 2,069,402.
Fasteners having drive sections with built-in washers or flanges are also known. At least some of these flanges have a wave or undulating configuration. In addition, retaining elements have been associated with some of the flanges. Prior art fasteners having flanges are disclosed in U.S. Pat. Nos. 3,877,339; 3,343,580; 3,056,443; 1,956,745; and 1,875,930.